I’ve been involved quite a lot in the reread of The Malazan Book of the Fallen(TMBotF) that is being done on Tor.com.
I’ve really enjoyed this series by Steven Erikson. When I saw that the reread was being done, I decided to give it a try and participate. So far, it has been a hoot. It’s kind of like a giant interactive con panel.
If you haven’t read TMBotF, give it a try. If it seems daunting, give the reread a try to help undaunt it.

I found the internet reaction to Deolalikar’s proposed proof that P<>NP (Deolalikar’s P vs NP paper)
fascinating. The paper is interesting in and of itself (if you like computational theory).
The massive number of notes and proof examination that the internet allowed was, to put it succinctly, really cool.

Edit:8/9/2012:
There hasn’t been much activity on this paper for quite some time. It would appear that it was a nice try, but contained several key errors. See the above link to a number of references.

Edit:7/16/2013:
Here is a link for the latest discussions on this. Essentially, there continue to be deep questions about the viability of the proof but there are many interesting areas that may be exposed.